With respect to semiconductor devices (power modules) in which a semiconductor element such as an IGBT (Insulated Gate Bipolar Transistor), etc., is mounted, adjustments are made whereby the heat generated at this semiconductor element is efficiently dissipated so as to keep it at or below a reference temperature even while heat is generated.
A conventional semiconductor device packaging structure will now be described based on FIG. 13. As shown in the diagram, a circuit board c is fixed on one side of an insulating substrate d comprising an aluminum nitride (AlN) substrate, or a pure aluminum substrate, etc. A semiconductor element a, which is electrically connected with a lead frame f that communicates with an external electrode, and the circuit board c are fixed by means of a solder layer b. A heat sink e for dissipating the heat from the semiconductor element a via the circuit board c (Q direction) is disposed on the other side of the insulating substrate d. A semiconductor device H is thus configured. It is noted that there are a wide variety of configurations for the semiconductor device in addition to that of the illustrated example, examples of which may include a configuration in which a cooler and the like are brazed below the heat sink, a configuration in which the illustrated device is potted with an encapsulating resin body, a configuration in which the semiconductor element is brazed onto the heat sink or the lead frame, etc.
In all of the configurations above, the fact remains that a semiconductor element is connected via a solder layer of a substrate, etc., and that the semiconductor device is of a multi-layered laminate structure of various constituent members. For example, in the illustrated example, the linear expansion rate (or the linear expansion coefficient) of the semiconductor element is approximately 3 ppm/K, the linear expansion rates of the circuit board and the insulating substrate are approximately 4 to 5 ppm/K, the linear expansion rate of the aluminum heat sink is approximately 25 ppm/K. Thus, the linear expansion rates vary significantly from constituent member to constituent member.
If the above-mentioned semiconductor device is to be mounted on hybrid vehicles or electric vehicles, it would have to be made sure that the semiconductor device would withstand, over extended periods, extreme temperature cycles. However, since the linear expansion rates vary significantly from constituent member to constituent member as mentioned above, if the members were to be joined with one another directly, thermal stress would be caused within the constituent members or at the joint interfaces among the constituent members due to differences in linear expansion caused by the change in temperature. Thus, the interior of the solder layer, which is weakest structurally and in strength among the constituent members, would be particularly prone to cracks, and this may become a significant factor in causing the durability of the semiconductor device to drop.
In particular, modem semiconductor devices have seen their miniaturization advance, and the heat generated by semiconductor elements has further increased with this miniaturization of semiconductor devices. Thus, in view of the fact that the amount of heat per unit area has increased, occurrences of cracks in the solder layer mentioned above have become even more pronounced.
Turning now to published prior art, Patent Document 1 discloses a method of detecting the occurrence of a crack based on a temperature difference by detecting the temperatures of respective sites by means of temperature detecting elements disposed at a center part of a semiconductor element and a peripheral end part thereof.
This detection method focuses on the fact that a site where a crack has occurred increases in thermal resistance, thereby impeding heat dissipation, and that the temperature rises as a result thereof. It is thus a method that determines that a crack has occurred at a given site when this site rises in temperature relative to other sites.
However, methods that determine the presence/absence of crack occurrences by measuring the temperature of a given site have the following problems.
Specifically, since heat diffuses readily, it may sometimes be determined that there is no temperature difference between a site where a crack has occurred and a site where no crack has occurred. Thus, so-called false detections occur readily, where a crack has actually occurred even though the measured temperature is low, or where there may actually be no crack even though the measured temperature is high.
Further, since it takes time for heat to be conducted, it is difficult to determine cracks in real time.